In U.S. Pat. No. 4,201,052, incorporated herein by reference, there is disclosed a pilot pressure operated high pressure load sensing valve system incorporated in a valve body designed to be mounted directly on an actuator to be controlled such as a hydraulic cylinder or hydraulic motor. The valve system accurately controls the position and speed of operation of the actuator.
In brief, the valve system disclosed in the aforementioned patent comprises an independent pilot operated meter-in element; a pair of load drop check valves; a pair of independently operated normally closed meter-out elements; a pair of load pressure responsive valves; and a pair of anti-cavitation valves. The meter-in element functions to direct fluid flow to one or the other of the actuator ports. The normally closed meter-out elements are associated with each of the actuator ports for controlling fluid flow from the port opposite to the actuator port to which the meter-in element is directing fluid. The meter-out elements function as variable orifices metering fluid between the appropriate actuator port and a low pressure zone such as a reservoir tank. Each of the meter-out elements has associated therewith the load pressure responsive valves which act on the meter-out elements in response to load pressure to enable the meter-out elements to also provide pressure relief protection. The anti-cavitation valves are associated with each of the actuator ports and are adapted to open the appropriate port to tank.
The valve system is directly mounted on the actuator port manifold and is supplied by one full flow high pressure line, a pair of pilot pressure lines, and a load sensing line. The operation of the valve system is controlled through the pilot lines from a manually operated hydraulic remote control valve. In the absence of a command signal from the hydraulic remote control, the meter-in element assumes a centered or neutral position with the check valves, the meter-out elements, the pressure responsive valves, and the anti-cavitation valves, all in closed position. In the neutral position, the valve system hydraulically locks the load in position. Fluid flow from the actuator is blocked thereby preventing uncontrolled lowering of an overhauling load in the event of a rupture of any of the connecting hydraulic lines. Since the valve system is a load sensing system, the pump output is made to match that which is required by the load. In contrast, in a non-load sensing system, the pump output may exceed that required by the load with the excess power being dissipated as heat.
In certain high inertial loads such as swing drives on an excavator which utilize rotary actuators, smooth stopping and starting of the load and accurate positioning of the load are very essential.
A hydraulic system providing for smooth stopping and starting and accurate positioning of the load under high inertial loads is disclosed in our copending U.S. application Ser. No. 264,342 filed May 18, 1981, now U.S. Pat. No. 4,407,122, wherein means are provided for sensing the pressure being directed to the actuator by the meter-in element and providing a feedback pressure using a small piston on the meter-in element opposing the pilot pressure tending to open the meter-in valve element.
Under certain conditions, it may not be possible or desirable to mount the valve system directly on the actuator. Such conditions may exist due to space limitations on the actuator or wherein it is desirable to limit the number of supply and pilot lines, such as to the topmost section of a telescoping boom or when a brake, such as in a winch-type application, is used for counterbalancing the load. Under these conditions, the valve system is mounted on the equipment remote from the actuator with a pair of lines running to the actuator port manifold.
In the latter situation it may be desirable to provide for controlled lowering or holding of the load at the actuator port manifold. In that case a conventional counterbalance valve is interposed between one of the actuator ports and the line leading from the valve system to the actuator port. In such an arrangement as disclosed our copending U.S. application Ser. No. 320,448 filed Nov. 12, 1981, now abandoned and refiled as pending U.S. application Ser. No. 605,607, filed Apr. 30, 1984, having a common assignee with the present application, the return flow from the actuator must pass through a normally open meter-out or exhaust element so as not to interfere with the desired control of the load through the counterbalance valve or brake. The normally open element is closed only when flow is delivered to the actuator in the opposite direction.
However, in the above described situation, when the meter-in valve is used as a flow control unit, it is usually difficult to obtain optimum stability of the load due to the high pressure gain in the outlet line of the meter-in valve.
Accordingly, it is an object of the present invention to provide a valve system of the aforementioned type which is operable in a counterbalance mode or with the use of external counterbalance valves or brakes with improved stability.
It is further an object of the invention to provide a hydraulic system having a proportional relationship between metered fluid flow and pressure in the output line of a flow control valve to maintain stability in the controlled lowering of an overhauling load.
It is another object of this invention to provide a hydraulic system which incorporates means for controlling an overhauling load and which hydraulic system has greater stability than prior hydraulic systems.
It is still another object of this invention to provide a hydraulic system incorporating a metering valve using pressure feedback to achieve system stability in the controlled lowering of an overhauling load.
In accordance with the invention, the meter-in element of the above described valve system is provided with a small feedback or load piston to establish a steady-state relationship between the metered flow and the outlet pressure of the valve system. The controlled pressure established by this steady-state relationship is used to control external counterbalance valves or to provide for the controlled release of a brake if it is desired to control an overhauling load by braking rather than hydraulic metering. The present invention also provides for operating one of the meter-out elements of the valve system as a counterbalance valve when it is desirable to mount the valve system directly to the actuator port manifold.
In accordance with another aspect of the invention, the load piston is not utilized but the circuit provides for counterbalance valves or brakes to control an overhauling load.